There are many mechanical devices for precisely detecting a displacement and a speed of a moving member to carry out feedback control. One example of such a device may be a lens barrel for an auto-focus camera. In a lens mirror barrel, a focusing mechanism for moving the lens, using an electric motor or a supersonic motor, is provided. To detect a rotation displacement of a rotation barrel constituting the focusing mechanism, a magnetic encoder is used. Patent Document 1 discloses a magnetic encoder used in a focusing mechanism, with external appearance thereof being shown in the perspective view in FIG. 18A. A magnetic sensor 210 is pressed onto a magnetic medium 150 having a curvature and provided along the mirror barrel 255. The magnetic sensor 210 comprises a magnetic sensor element 240 and a pressure spring 220. As is obvious from FIG. 18A, the magnetic sensor element 240 is very thin in view of the size of the surface of the magnetic medium, the surface being opposed to the magnetic sensor element 240. That is, the magnetic sensor element 240 is required to be thin as the space within the mirror barrel is limited. Therefore, a magnetic encoder is used oftener than an optical encoder, which is not readily formed thin. An output of the magnetic sensor element 240 is fed back, and the mirror barrel 255 is driven for focusing, using the motor 260.
For highly accurate detection of displacement, a magnetic encoder is required to have high resolution. Resolution can be expressed in the form of a magnetic pole pitch of a magnetic medium. A magnetic pole pitch, conventionally being 30 to 50 μm, is recently required to be 10 μm to 20 μm or even smaller than 10 μm. In pursuit of higher resolution, influence of an interval, or a gap, between a magnetic medium and a magnetic sensor element becomes larger, and elimination of gap variation is accordingly required. In view of the above, advantageously, an arrangement in which the magnetic medium is placed in contact with the magnetic sensor element and slid together is often employed.
To help understanding of a positional relationship between the magnetic sensor element and the magnetic medium in this specification, vertical coordinates having the X axis in the direction in which the magnetic sensor element reciprocatively slides relative to the magnetic medium, the Y axis in the direction along the magnetic medium surface and perpendicular to the direction in which the magnetic sensor element reciprocatively slides relative to the magnetic medium, and the Z direction perpendicular to the magnetic medium surface are assumed. Further, a point (a pressure point) at which a force for pressing the magnetic sensor element onto the magnetic medium is applied is defined as the origin of the X axis, and a displacement from the X axial origin in the X direction, of a relative position between the magnetic sensor element and the magnetic medium is defined as an X offset. Still further, in order to describe relative posture of the magnetic sensor element relative to the magnetic medium surface, an angle by which the magnetic sensor element surface tilts around the X axis is defined as a pitch angle, and an angle by which the magnetic sensor element surface tilts around the Y axis is defined as a roll angle, in which the roll angle and the pitch angle with the sliding surface in parallel to the X-Y plane surface are defined as 0 degree, respectively.
FIG. 18B is a side view of a structure of a pressure spring 220 for evenly pressing the magnetic sensor element 240 onto the magnetic medium 150 when assembling in order to ensure a stable pitch angle of the magnetic sensor element 240 when sliding. The magnetic sensor element 240 is mounted in the holder 222, which can rock relative to the pressure spring 220, using the rocking central axis on the back surface of the holder 222 as a fulcrum. Through rocking, the magnetic sensor element 240 can stay closely in touch with the magnetic medium 150 via the spacer 246 even though the distance between the fixing portion of the pressure spring 220 and the magnetic medium should vary. As the magnetic sensor element 240 rocks with the rocking center in substantially parallel to the displacing direction of the magnetic medium 150, the magnetic sensor element 240 stays closely in touch with the magnetic medium 150, having the spacer 246 in-between. This enables accurate detection of an amount by which the magnetic medium 150 has moved (that is, an amount by which the focusing lenses have moved). The rocking center serves as a rocking fulcrum, i.e., a pressure point 224 where the magnetic sensor element 240 is pressed onto the magnetic medium 150. An output of the magnetic sensor element 240 is got through an FPC (Flexible Print Circuit) 244.
Patent Document 2 discloses a structure of a magnetic encoder having a single leaf spring which has a pressure function and a uniform pitch angle maintaining function. As shown in the perspective view in FIG. 19A, the leaf spring 320 holds a magnetic sensor element 340, using a sensor holding portion 322; a first arm portion 334, a connection unit 335, and a second arm portion 336 of the leaf spring 320 together support the sensor holding portion 322; and a fixing portion 326 is fixed to a mounting pedestal 360. As shown in the side view in FIG. 19B, even when the distance between the fixing portion 326 of the leaf spring 320 and the magnetic medium 150 should vary, the first arm portion 334 and the second arm portion 336 are flexed in opposite directions, whereby the pitch angle can be maintained unchanged. To respond to a request for higher accuracy, however, gap variation due to a roll angle and an X offset between the magnetic medium 150 and the magnetic sensor element 340 remain a major problem to be solved.
Patent Document 3 discloses a magnetic sensor holding structure capable of reducing gap variation due to a roll angle and an X offset of a magnetic sensor element. As shown in the perspective view in FIG. 20, Patent Document 3 proposes a very narrow magnetic sensor element 440 having a width W in the slide direction of the magnetic sensor element 440 being 0.04 to 0.3 mm, i.e., twice to fifteen times the magnetic pole pitch. Note that FIG. 20A is a perspective view showing the entire magnetic sensor 410, in which the magnetic sensor element 440 is opposed to the magnetic medium 150. FIG. 20B is a perspective view showing a structure in which the magnetic sensor element 440 is mounted on the suspension 420 at the tip end of the magnetic sensor 410; FIG. 20C is an enlarged perspective view of the magnetic sensor element 440. The width w, in the slide direction, of the magnetic sensor element 440 in contact with the magnetic medium 150 is set to as small as 0.3 mm or smaller to reduce gap variation due to an X offset and a roll angle to thereby stabilize a signal output amplitude.
Patent Document 4 discloses a magnetic sensor holding structure having a single leaf spring which has a pressure spring function and a gimbal spring function which follows pitch angle and roll angle variation. FIG. 21A is a perspective view showing external appearance of the structure. The magnetic sensor holding structure is for use in a hard disk drive (HDD), and having a quadrate swirling leaf spring 520 which comprises a slider holding portion 522 for holding a floating head slider 540, a fixing portion 526, and a swirling elastic portion 534. The swirling elastic portion 534 extends from a substantial end portion of each of the four edges of the fixing portion 526 so as to surround about three-fourths of the periphery of the slider holding portion 522 and reach the substantially middle portion of each edge thereof. To ensure a longer swirling elastic portion 534, the swirling elastic portion 534 extends from an end portion of the fixing portion 526 to the middle portion of each edge of the slider holding portion 522. With an elongated swirling elastic portion 534, elasticity in the roll direction and that in the pitch direction can be reduced, so that followability of the floating head slider 540 relative to fine convexes and concaves formed on the magnetic medium surface can be enhanced. However, as the elasticity in the roll direction and that in the pitch direction are reduced, a force (a load) for pressing the floating head slider 540 onto the magnetic medium 150 cannot be obtained. In addition, recording and reproduction while sliding the magnetic medium 150 and the floating head slider 540 is not possible in view of tracking accuracy.    Patent Document 1: Japanese Patent Laid-open Publication No. 2000-205808    Patent Document 2: Japanese Patent Laid-open Publication No. 2003-344105    Patent Document 3: Japanese Patent Laid-open Publication No. 2006-64381    Patent Document 4: Japanese Patent Laid-open Publication No. Sho 63-149888